Multifrequency video displays or monitors are used for both High Definition Television (HDTV) and National Television Standards Committee (NTSC) television viewing and for computer applications. At the current state of the art, charge-coupled device (CCD) displays tend to be costly, or unavailable, especially in large sizes, and in general are not as bright as kinescope or picture-tube types of displays. Plasma displays are not common. Thus, the kinescope display is in common use.
Kinescope displays are ordinarily scanned by means of inductive or electromagnetic yokes near the neck of the kinescope, to which both vertical and horizontal scan currents are applied, to create magnetic fields which deviate one or more electron beams traversing the tube to the phosphorescent display screen. National Television Standards Committee (NTSC) standards for conventional television specify a horizontal scanning frequency of about 15,734 Hz, and a vertical scan frequency of 60 Hz. A large body of art has grown up around vertical and horizontal deflection circuits. Because of the relatively high horizontal scan frequency and significant power involved in performing horizontal scan, the horizontal deflection scanning circuits commonly operate in a resonant retrace mode, in which currents and the associated power are recirculated back to the power source for re-use during the next following scan cycle.
The amplitude of current circulating in a resonant horizontal deflection circuit is mainly determined by the value of the B+ voltage. It is also determined, at least in part, by the desired scan frequency. For a given horizontal deflection system with a fixed value of kinescope ultor (High) voltage, a fixed amount of overscan, and a horizontal deflection yoke having a fixed inductance, the product of the scan time multiplied by the deflection circuit B+ tends to be a constant. Thus, the value of energizing voltage or B+ applied to the horizontal deflection circuit multiplied by the scan time is desirably constant. In the past, many video display systems were designed to apply a constant B+ to the horizontal deflection system.
With the advent of HDTV, many different video formats are available to the consumer. Some of these formats have vertical and horizontal deflection frequencies which differ from those of conventional NTSC broadcast television signals. The higher definition associated with HDTV implies a higher horizontal deflection frequency than for standard-definition television. As the horizontal frequency increases, the scan time must decrease, unless the frequency difference is very small and can be taken up in the retrace time. For most television systems in which the horizontal scanning frequency is varied, the retrace time is held constant, and the scan time is varied. Thus, as the horizontal scanning frequency is increased, the scan time is decreased, and the scan B+ is also increased so that the product of the scan time multiplied by the B+ is a constant.
Many present-day television display devices, including television receivers, derive their scan B+ voltage from a switch mode power supply (SMPS) including inductive elements and a transformer having a secondary winding. The scan B+ is generated from the scan secondary winding by rectification of SMPS pulsatory signals and subsequent filtration. In many such display devices, in order to avoid the need for plural power supplies, the SMPS must also supply auxiliary or ancillary voltages, for operation of systems or circuits other than the horizontal deflection system. In a television receiver, for example, the ancillary systems may include RF and IF processors, video and sound processors, convergence, and others. These ancillary systems ordinarily require that their energizing voltages remain constant, regardless of the horizontal deflection frequency which happens to be in use. The ancillary energizing voltages may be derived from secondary windings of the SMPS transformer other than the one from which the horizontal scan or deflection B+ is derived. However, simply deriving the ancillary energizing voltages from a separate secondary winding will not guarantee that the ancillary energizing voltage does not change. Since the number of turns per winding in the SMPS transformer is fixed, changing the horizontal scan B+, without more, also changes the ancillary energizing voltage.
Improved video display arrangements are desired.